1. Field of the Invention
This invention relates to lubricating oil compositions, their method of preparation and use. Specifically, this invention relates to lubricating compositions that contain an antioxidant additive comprising a metal-free sulfur-containing compound, a hindered amine and an aromatic amine.
2. Description of the Related Art
Oxidation is a major cause of the breakdown of lubricants. This results in a shortened lifespan of the lubricant, requiring more frequent changes, especially in demanding environments such as internal combustion engines.
Antioxidants have therefore played an important role as additives in lubricants in order to extend their useful life. Aryl amines (also called aromatic amines), especially secondary diarylamines, e.g., alkylated diphenylamines, phenothiazines, and alkylated N-naphthyl-N-phenylamines have been important additives to lubricating compositions. Also important have been phenolic compounds in retarding oxidation.
Other combinations of antioxidants have also been used. U.S. Pat. Nos. 5,073,278 and 5,273,669 to Schumacher et al. disclose the synergistic combination of aromatic amines and hindered amines in a lubricating oil. U.S. Pat. No. 5,268,113 to Evans et al. discloses the combination of a hindered amine with phenolic compounds.
Sulfurized organic compounds have also been shown to have antioxidant activity. U.S. Pat. No. 4,880,551 to Doe and U.S. Pat. No. 6,743,759 to Stunkel disclose the synergies between an ashless dithiocarbamate with triazole compounds. U.S. Pat. No. 6,806,241 discloses the synergy between an ashless dithiocarbamate, a molybdenum compound, and an alkylated diphenylamine.